Immunization with live, attenuated virus vaccines has consistently provided robust protection in macaques against challenge with pathogenic SIV. However, the promise of this approach continues to be tempered by persistent concerns over the risk of using a live HIV-1 in humans, regardless of its degree of attenuation. Thus, we have turned our attention to studying the immune correlates of protection in this model, with the goal that this information can be applied towards the development of safer alternatives. In the past year, we have initiated a study to evaluate the correlates of immune protection in rhesus macaques immunized with a live, attenuated SIV. In this project, we will extend our current findings by specifically addressing the correlates of immune protection in a series of observation and interventional studies. The specific aims are as follows: 1. Observational Studies (a) Identification of a temporal correlation between the induction of various immune responses and the onset of protection in rhesus macaques immunized with SIV/mac/delta/nef. (b) Phenotypic and genotypic characterization of he breakthrough virus in macaques that fail to be protected by SIV/mac/delta/nef. (c) Analysis of the level of replication and tissue localization of SIV/mac/delta/nef in vivo. 2. Interventional Studies (a) Re-challenge macaques protected by SIV/mac.delta/nef with a pathogenic (89.6P) and a non-pathogenic (89.6) SHIV to examine the role of env in protection. (b) Treat SIV/mac/delta/nef-protected macaques with a potent anti- retroviral agent (PMPA) to determine if protection persists. (c) Determine whether immunized macaques are protected from mucosal challenge with pathogenic SIV, and if so, which step in the early events of infection is blocked by immune responses elicited by SIV/mac/delta/nef. (d) Challenge SIV/mac/delta/nef-protected monkeys with a SIV engineered to express ICP47, an inhibitor of TAP (transporter of peptides for MHC presentation), to assess the role of cytotoxic T lymphocytes (CTL) in mediating protection.